Combination instruments for drafting and geometric construction are well known and have been in use for many years. For example, a U.S. Pat. No. 2,054,420 of Hochman, discloses a combination drafting implement that includes a ruler, a protractor and a compass. A protractor includes a generally semi-circular arc plate with the usual angle scale markings thereon. A protractor and scale elements are preferably distinct so that each can be used independently of the other according to conventional practices.
A more recent drafting instrument is disclosed in a patent of Concari et al., U.S. Pat. No. 5,426,859. As disclosed therein an instrument for performing various drawing and measurement functions is disclosed. The instrument includes an elongated plate having a straight edge and a measurement region. A protractor plate is connected to the elongated plate and cooperates with a rotatable center dial for the drawing and measuring of angles. The instrument also preferably includes holes through the elongated plate. The holes are configured to receive a marking instrument so the device can be used as a compass. Additionally, an attachment region allows the instrument to be held in a conventional ring binder.
Finally, a U.S. Pat. No. 6,405,443 of Thorn et al. discloses a geometric construction device. The device forms circles or arcs with a marking instrument and simultaneously measures angles. The device includes a radius arm and a rotatable member. The radius arm includes means for receiving the marking instrument and a disc having a pivot point and being rotatable about the pivot point. The disc is used to attach the rotatable member to the radius arm. The rotatable member in combination with the radius arm forms a 360° protractor. The rotatable member has a plurality of radius holes in it for receiving the marking instrument. These radius holes may be used to form circles or arcs with the radius being less than or equal to the radius of the disc. The 360° protractor is capable of measuring degrees between 0° and 360° while circles or arcs are being formed.
Notwithstanding the above it is presently believed that there is a need and a potential commercial market for an improved set of interacting instruments. There should be a need for such sets because they include a ruler, an adjustable triangle assembly, a compass and a protractor. Further, it is believed that such devices can be manufactured and sold at a competitive price, are durable and easy to use to construct or analyze mathematical figures.